PG&E's Diablo Canyon nuclear power plant in Avila Beach, Calif. on Friday, May 26, 2006. The two spent fuel storage pools are nearing its capacity of 2,648 cells so plant officials are constructing a dry cask storage area to hold future radioactive fuel cell waste.
PAUL CHINN/The Chronicle less

PG&E's Diablo Canyon nuclear power plant in Avila Beach, Calif. on Friday, May 26, 2006. The two spent fuel storage pools are nearing its capacity of 2,648 cells so plant officials are constructing a dry cask ... more

America's top nuclear regulator said Wednesday that if anyone wants to study in depth the offshore earthquake faults near PG&E's Diablo Canyon Power Plant, there is no substitute for the environmentally destructive tests blocked by California officials in November.

Allison Macfarlane, chairwoman of the Nuclear Regulatory Commission, emphasized that her agency had not asked Pacific Gas and Electric Co. to conduct those tests. Nor would the tests be required to extend Diablo Canyon's operating licenses from the commission.

But the tests, which involve blasting the seabed with air guns that create intense sound waves, are the only way to study the fault lines several miles into the rock, she said. Other methods, which PG&E has already used, only extend "some hundreds of meters" below the seabed.

"This would take you further," said Macfarlane, who holds a doctorate in geology from the Massachusetts Institute of Technology. "This would take you kilometers, which would be nice to know. So without this, you're just in more uncertain territory."

PG&E has been under pressure from state officials to conduct more thorough seismic tests around Diablo, pressure that increased after the U.S. Geological Survey found another fault line near the plant four years ago. But in November, the California Coastal Commission blocked PG&E's testing plan, citing the damage it could do to fish, sea lions and a rare type of porpoise living nearby.

Macfarlane made her comments in an interview with The Chronicle after touring Diablo Canyon and California's only other commercial nuclear plant, the San Onofre Nuclear Generating Station in San Diego County. San Onofre shut down nearly a year ago following a small leak of radioactive steam. The leak was traced to prematurely worn-down tubes within a steam generator.

Southern California Edison, which operates the plant, wants to restart the facility, despite the objections of some neighbors and environmentalists who want to keep it closed. Macfarlane said the commission would only allow a restart if Edison can do it safely.

"Our mission is to protect public health and safety, protect the environment, and we will carry out that mission there," she said.

PG&E had been seeking an extension of Diablo Canyon's operating licenses from the NRC, potentially extending the plant's life through 2045. But after the 2011 meltdown at Japan's Fukushima Dai-ichi nuclear plant, PG&E asked the commission to place the renewal process on hold until the company could finish its seismic studies.